This invention relates to an illuminating apparatus such as a slide projector, a liquid-crystal video projector or an OHP sheet projector that are suitable for illuminating objects, especially those having a rectangular or square shape.
Conventional illuminating apparatus of the type contemplated by the present invention are available as a system in which a light source such as a halogen lamp, a xenon lamp or a metal halide lamp is combined with a parabolic or spheroidal mirror and a condenser lens (see, for example, Chapter 3, Section 1 of "Designing Projection Televisions", Trikeps Publishing Company). However, these conventional systems have had the problem that the light issuing from the light source in directions other than those pointed to the parabolic or spherical mirror is not effectively used for illumination, thereby lowering the efficiency of utilization of the light emitted from the light source. (On the pages that follow, the proportion of emitted light that is reflected by the parabolic or spheroidal mirror and thereafter collected is sometimes referred to as the "efficiency of utilization of emitted light").
An illuminating apparatus intended to solve this problem is described in Japanese Patent Laid-Open Publication (kokai) Hei-3-168629. FIG. 10 is a simplified cross-sectional view showing diagrammatically the composition of that apparatus. As shown, a light source 1 is positioned at the first focal point of a spheroidal mirror 2. A spherical mirror 3 whose radius of curvature is equal to the focal length of the spheroidal mirror 2 is positioned in such a way that-the center of curvature coincides with the first focal point of the spheroidal mirror 2. An aperture 31 of a desired size is provided in the central portion of the spherical mirror 3.
A light beam 61 from the light source 1 that has reached the surface of the spherical mirror 3 is reflected, collected at the light source 1 and thereafter reaches the surface of the spheroidal mirror 2. This beam is collected at the second focal point of the spheroidal mirror 2 together with a light beam 62 from the light source 1 that has directly reached the surface of the spheroidal mirror 2.
The light collected at the second focal point of the spheroidal mirror 2 passes through the aperture 31 in the spherical mirror 3 and is incident on a condenser lens 4, from which it emerges as parallel or slightly convergent light rays that illuminate a rectangular object 5.
As is clear from FIG. 10, the radius of curvature of the spherical mirror 3 is equal to the focal length of the spheroidal mirror 2 and the second focal point of the spheroidal mirror 2 lies on the plane of the spherical mirror 3. Because of these features, it has been possible to reduce the size of aperture 31 to a very small scale. Hence, practically all part of the light emitted from the light source 1 can be collected at the second focal point of the spheroidal mirror 2, thereby producing illuminating rays that permit very efficient utilization of emitted light.
Another illuminating apparatus that has been proposed to solve the aforementioned problem of the prior art is described in Japanese patent Laid-Open Publication (kokai) HEI No. 4-063321. FIG. 11 is a simplified cross-sectional view showing diagrammatically the composition of the apparatus. As shown, a light source 1 is positioned at the first focal point of a spheroidal mirror 2 and a spherical mirror 3 is positioned in such a way that its center of curvature coincides with the first focal point of the spheroidal mirror 2. A rectangular aperture 31 of a desired size is provided in the central portion of the spherical mirror 3. The spheroidal mirror 2 is truncated by the lines of intersection created when a quadratic prism having the same cross-sectional shape as the aperture 31 is inserted into that aperture. The peripheral portion of the mirror 2 assumes a rectangular shape when it is seen at infinity on the optical axis 8 passing through the central portion of the exit plane.
A light beam 61 from the light source 1 that has reached the surface of the spherical mirror 3 is reflected, collected at the light source 1 and thereafter reaches the surface of the spheroidal mirror 2. This beam is collected at the second focal point of the spheroidal mirror 2 together with a light beam 62 from the light source 1 that has directly reached the surface of the spheroidal mirror 2.
If the second focal point is far more distant than the rectangular object 5 to be illuminated as in the apparatus shown in FIG. 11, the light collected at the second focal point emerges as slightly convergent rays which are capable of illuminating the rectangular object 5.
The system under consideration produces practically rectangular illuminating rays and, therefore, if the peripheral shapes of the aperture 31 and the spheroidal mirror 2 are set in such a way that a cross section of the illuminating rays reaching the rectangular object 5 has the same dimensions as the object, almost all of the illuminating rays can be used to illuminate the object 5. (On the pages that follow, the proportion of the area of the patterned shape that is illuminated with the illuminating rays and which is occupied by the area of the object being illuminated is referred to as the "efficiency of illumination").
The system described in Japanese Patent Laid-Open Publication (kokai) Hei-3-168629, supra, achieves very efficient utilization of emitted light but, on the other hand, the peripheral shape of the spheroidal mirror 2 is circular and, hence, the pattern of illumination 72 with the illuminating rays 71 that reach the object 5 is circular as shown in FIG. 12. In contrast, the shape of the object 5 is not necessarily circular, so the pattern of illumination 72 with the illuminating rays 71 must be in a circular form of a size that is at least large enough to be circumscribed with the object 5. Under the circumstances, that portion of the light which illuminates outside the object 5 is not effectively used, thus lowering the efficiency of illumination.
The system described in Japanese Patent Laid-Open Publication (kokai) Hei-4-063321, supra, produces the illuminating rays 71 having a rectangular pattern of illumination 72 and the peripheral shapes of the aperture 31 and the spheroidal mirror 2 can be set in such a way that the pattern of illumination 72 with the illuminating beams 71 reaching the rectangular object 5 has the same dimensions as said object. Hence, almost all of the illuminating rays can be effectively used to achieve very efficient illumination.
However, in view of the slightly convergent nature of the illuminating rays 71, the aperture 31 in the spherical mirror 3 must be larger than the object 5. In other words, the size of the aperture 31 increases considerably, and so does the amount of a light beam 64 from the light source 1 that leaks directly through the aperture 31 without contributing to the intended illumination. As a result, the efficiency of utilization of emitted light is lowered.
Under the circumstances, the present inventors thought of altering the peripheral shape of the spheroidal mirror in the system described in Japanese Patent Laid-Open Publication (kokai) Hei-3-168629, and they reviewed the case where the spheroidal mirror 2 was redesigned to have a rectangular periphery (as seen at infinity), which is the shape of the spheroidal mirror used in the system described in Japanese Patent Laid-Open Publication (kokai) Hei-4-063321, supra (see FIG. 11).
This modified system (hereunder sometimes referred to as the "reviewed case") achieved very efficient utilization of emitted light as in the case described in Japanese Patent Laid-Open Publication (kokai) Hei-3-168629. However, as it turned out, the magnification achieved by the spheroidal mirror 2 differed between the area near the optical axis and the peripheral portion and, hence, the pattern of illumination 72 with the illuminating rays 71 as collimated by the condenser lens did not come out in a rectangular shape as in the case of Japanese Patent Laid-Open Publication (kokai) Hei-4-063321 but was distorted in a pincushion form as shown in FIG. 6. This has caused the problem that if the pattern of illumination 72 with the illuminating rays 71 is adjusted to be of a size that is circumscribed with the object 5, the rays of light illuminating outside the object 5 are not effectively used, thus failing to achieve efficient illumination.